The overall objective of this research proposal is to determine the molecular mechanisms by which photoreceptor cell fate is specified. This will be accomplished by using a combination of molecular biological and genetic techniques. The system of study will be the compound eye of the fruit fly Drosophila melanogaster. In recently published work, we identified and characterized a novel pattern of visual pigment gene expression in the fly eye. By manipulating the eye genetically we have shown that the expression of visual pigment genes in one photoreceptor cell type. It appears that one cell in each ommatidium (R7) adopts one of two different cell fates in a stochastic manner, and then communicates this decision (inductively) to the adjacent R8 cell. These events serve to coordinate the expression of visual pigments in these two cells, and produce two type of optical units within the eye that have distinct spectral sensitivities. In this proposal, we present a series of experiments to rigorously test this model, by examining aspects of opsin gene expression, studying the promoters which regulate their expression, and conducting genetic screens to identify genes which are required for the specification of photoreceptor cell fate and the regulation of opsin gene expression. These studies are of fundamental importance in understanding how the compound eye is formed. The differentiation of photoreceptors into populations having different spectral sensitivities is the basis for color discrimination. Signal trafficking within the nervous as a whole is also dependent upon the generation f diverse neuronal populations having different sensitivities and transmitter phenotypes. Because the fly eye has become a key model system in Developmental Biology, the elucidation of novel signaling and cell-fate determination pathways within this system will serve as a framework to understand more complex developmental events within the nervous system of other organisms.